1. Field of the Invention
The present invention relates to an optical wavelength converter device, and more particularly to an optical wavelength converter device for converting two fundamental waves of different wavelengths applied to the device into a wave having a frequency equal to the sum of the frequencies of the fundamental waves, a wave having a frequency equal to the difference between the frequencies of the fundamental waves, or two or more of wavelength-converted waves respectively having the sum frequency, the differential frequency, and a second harmonic.
2. Description of the Prior Art
Various attempts have heretofore been made for applying light as a fundamental wave to a nonlinear optical material to convert the fundamental wave to a second harmonic having a wavelength which is 1/2 of the wavelength of the fundamental wave. It has also been attempted to apply two fundamental waves having different wavelengths .lambda..sub.1, .lambda..sub.2 to produce a wave having a wavelength .lambda..sub.3 and a sum frequency (1/.lambda..sub.3 =1/.lambda..sub.1 +1/.lambda..sub.2) or a wave having a wavelength .lambda..sub.3 and a differential frequency (1/.lambda..sub.3 =1/.lambda..sub.1 -1/.lambda..sub.2). Examples of such optical wavelength converter devices using a nonlinear optical material for wavelength conversion include an optical wavelength converter device of the bulk crystal type and an optical wavelength converter device of the three-dimensional waveguide type which is disclosed in page 1234 and following pages of "OYO BUTURI" (a monthly publication of the Japan Society of Applied Physics), Vol. 49 (1980).
With the optical wavelength converter device of the bulk crystal type, however, the power density of fundamental waves to be applied to the device cannot be increased sufficiently, and the interaction length of the device cannot be large in view of the requirements for extracting a converted wave. Therefore, the efficiency of wavelength conversion is very low.
The optical wavelength converter device of this type is designed to achieve phase matching between the fundamental waves and the converted wave by utilizing the birefringence of a crystal. The condition for achieving the phase matching is indicated by: EQU .vertline.K.sub.1 .+-..vertline.K.sub.2 =.vertline.K.sub.3
where .vertline.K.sub.1, .vertline.K.sub.2 represents the wave number vectors of first and second fundamental waves, and .vertline.K.sub.3 the wave number vector of a wave with a sum or differential frequency. Assuming that the refractive index of the device for the first fundamental wave in the direction of polarization is indicated by n.sub.1, EQU .vertline..vertline.K.sub.1 .vertline.=(2.pi.n.sub.1)/.lambda..sub.1, EQU and likewise, EQU .vertline..vertline.K.sub.2 .vertline.=(2.pi.n.sub.2)/.lambda..sub.2, EQU .vertline..vertline.K.sub.3 .vertline.=(2.pi.n.sub.3)/.lambda..sub.3,
(.lambda..sub.1 &lt;.lambda..sub.2). In order to meet the foregoing phase matching condition, it is necessary that the refractive indexes n.sub.1, n.sub.2 (in the direction of polarization of the second fundamental wave) and the refractive index n.sub.3 (in the direction of polarization of the converted wave) be of desired values with respect to the wavelengths .lambda..sub.1, .lambda..sub.2, .lambda..sub.3. Therefore, the optical wavelength converter device can be used only in a highly limited range of wavelengths.
There has been reported only one example of an optical wavelength converter device of the three-dimensional waveguide type, in which a substrate is made of LiNbO.sub.3 and a wave having a differential frequency is produced from two fundamental waves with different frequencies. The condition for achieving phase matching between the applied fundamental waves is expressed by: ##EQU1## where n.sub.eff.sup..omega.1, n.sub.eff.sup..omega.2 represent the effective refractive indexes of the optical waveguide with respect to the first and second fundamentals, and n.sub.eff.sup..omega.3 the effective refractive index of the optical waveguide with respect to the wave with the differential frequency. Unlike the bulk-crystal-type optical wavelength converter device, it is possible to increase the power density of the fundamental waves applied to the optical wavelength converter device of the three-dimensional waveguide type. It has been theoretically shown that using a device having a length of 1 cm, a wavelength conversion efficiency of a few % can be achieved with an input power of 100 mW. Nevertheless, since the above phase matching condition cannot be satisfied unless the temperature of the device is controlled with an accuracy of 0.1.degree. C. or less, the optical wavelength converter device of this type has not yet been put to use.
In order to obtain a converted wave having a desired wavelength (and a differential frequency), it is necessary to control the refractive indexes of the optical waveguide. In the optical wavelength converter device of this type employing a substrate of LiNbO.sub.3, however, the refractive indexes can only be controlled by a diffusion process or a proton exchange, and hence the degree of freedom available in designing the optical waveguide is low.
According to the one known example of the optical wavelength converter device of the three-dimensional waveguide type, which was mentioned above, only a wave having a differential frequency is extracted. No optical wavelength converter device of the bulk crystal type has been proposed which can simultaneously produce waves having second harmonics of two respective fundamentals, or which can simultaneously produce one or both of the waves having second harmonics and one or both of waves having sum and differential frequencies.